Production of dichlorodiphenyltrichlorethane



United States PatentjOi PRODUCTION F DICHLORODDHENYLTRI- CHLORETHAN EFrank J. Handel, Arcadia, Califi, assignor to Wigton- AbbottCorporation, Ilainfield, N. 3., a corporation of New Jersey No Drawing.Application July 15, 1955, Serial No. 522,367

1 Claim. (Cl. 260-649) This invention relates to a process for preparingdichlorodiphenyltrichlorethane, hereinafter referred to as DDT, and moreparticularly to an improved procedure for preparing DDT in a finelydivided and friable form. This is a continuation-in-part of applicationSerial No. 289,210, filed May 21, 1952, and now abandoned.

The production of DDT has in the past been complicated by the fact thatthe product generally obtained is of a waxy nature which cannot bereadily ground or subdivided. Various procedures have been proposed forovercoming this difliculty, as for example, by controlled cooling ofmolten DDT, and While such cooling process yields a more or less friableproduct, it nevertheless yields a product which must be ground in orderto produce a uniform finely divided solid.

The production of DDT has been further complicated by the fact that DDT,as commercially prepared, comprises mostly a mixture of two isomericmaterials, i. e. 2,2-bis (p-chlorophenyl)-l,l,l-trichlorethane and 2-0-chlorophenyl 2 p chlorophenyl 1,1,l-trichlorethane, hereinafter referredto as the p,p and o,p isomers. From the standpoint ofinsecticidalproperties, the p,p' DDT is the most effective. In addition, the p,p'DDT has a higher melting point, melting at about l08.5-109.0 C., whereasthe o,p DDT melts at about 74.0-74.5 C. In commercial products, the p,pisomer and the o,p isomer are generally present in the proportions ofabout 70 to 75 percent of the former to about 19 to 21 percent of thelatter. If the proportion of the o,p isomer exceeds the amount specifiedas frequently occurs, the product has a marked tendency to assume theundesirable waxy characteristic which interferes with grinding andfinishing operations.

I have discovered in accordance with the present invention a procedurewhereby molten DDT, even if it contains an excessive amount of theundesired 0,p' isomer can be converted directly to a finely divided,friable solid product. Regarded in certain of its broader aspects, mynovel process comprises mixing together with agitation the molten'DDTand a mixture of organic liquid in which both isomers of. DDT are fairlyreadily soluble at a higher temperature,"however, below the boilingpoint of the mixture, but .diificultly soluble at lower temperatures,introducing the DDT at such a rate as to maintain a supersaturatedcondiion whereby molten DDT will go into solution and at the same timefinely divided, solid DDT will fall out of solution, separating thesolids which thus fall out of solution, and drying the same to removeentrained organic liquid.

The process lends itself either to continuous operation in which aportion of the reaction mixture is constantly removed for separation ofthe finely divided solid DDT, the separated mixture of organic liquidsbeing recycled and contacted with fresh molten DDT; and to batchwiseoperation wherein a quantity of organic liquids is combined with moltenDDT under vigorous agitation until the desired degree of supersaturationis achieved and the 2,786,084 Patented -Mar. 1 9, 1957 entire mass orreaction mixture is then treated forseparation of finely divided solidDDT. The continuous process is particularly advantageous as the finelydivided solid DDT can readily be recovered by continuous rotary drum,vacuum filters or continuous centrifugal filters. In each instance, theorganic liquids can readily be recycled and the air or other dryingmedium can be continuously treated for recovery of organic liquidtherefrom.

While there are a number of mixtures of organic liquids which have thedesired properties of dissolving the two isomers of DDT readily at ahigher temperature while dissolving the isomers only with difiiculty atlower temperatures, the mixture of organic liquids which I findpreferable in theprocess consists of methyl alcohol and acetone.

In my solvent mixture, the extent to which DDT can be dissolved thereinis' of primary importance. Thus, for example, methyl alcohol alonewithout admixture of acetone does not provide a suitable solvent as thesolubility of DDT in methyl alcohol is practically nil even at theboiling point of methyl alcohol. If methyl alcohol were used alone in myprocess, suspended particles of molten DDT would, instead of going intosolution, become surfused or super-cooled and inwardly would form gummyor taify-like masses quite unlike the friable dry produr" which myprocess is adapted to produce. By the addi tion of about 15 percentacetone, the solubility of DUI in the methanol acetone mixture isincreased sufliciently so that DDT will go into solution from the moltenstate and be thrown out as a finely divided solid from the resultingsupersaturated solution.

The mechanism of my process may be explained as follows:

During the rapid and initimate contacting of the solvent with moltenDDT, both isomers p,p' and o,p' go into solution until the solvent getssaturated at a given temperature. Further admixing of molten DDT stilldoes not result in throwing the solids out of solvent; this is due tothe supercooling eliect of the saturated solution. After reaching acertain point of supersaturation, DDT starts falling out in finecrystalline or amorphous form. This metastable condition of thesupersaturated DDT solutions is so persistent that although solids ofDDT start to fall out of the solution, fresh molten DDT is at the sametime going into solution to maintain a supersaturated condition. In thisconnection, it should be noted that the phenomenon of additional DDTgoing into solution to maintain the supersaturated condition is peculiarto molten DDTv The solid DDT, once separated from the supersaturatedcondition, does not go back into solution under the prevailingconditions.

The molten DDT and solvent are combined under conditions such that theresulting mixture is maintained preferably at a temperature between 0 C.and 50 C. In this way, a product of high setting point is obtained, orin other words, a product having a maximum portion of the p,p isomer ofDDT. As the temperature and reaction mixture is lowered below 50 C., aproduct of progressively lower setting point is obtained. Thus, bycontrolling 'the temperature of the reaction mixture, it is possible tocontrol the extent to which the preferred p,p' isomer of DDT isconcentrated in the recovered solid, and the o,p isomer is removed inthe separated solvent. When an appreciable amount of o,p' isomer is thusbeing separated in the solvent, it is possible to remove this isomer atleast partially from the solvent before recycling, as for example bycooling the solvent to a lower temperature at which the o,p is thrownout as solid material.

It is of primary importance both for facilitating rapid solution ofmolten DDT and for producing a finely di- 3 vided product as itseparates from the supersaturated solution, that thorough agitation ofthe molten DDT and solvent be provided as they are brought together.This agitation can be effected either by means of a rotary mixer or bypassing the mixture through a series of baffles or orifices to provideturbulence in the mixture.

The finely divided, solid DDT prepared by my process will vary in bulkdensity depending upon the amount of of the reaction mixture remained inthe reaction vessel from one batch to the other.

The following is a tabulation of results obtained in the treatment ofeight successive quantities of DDT after equilibrium conditions had beenreached, i. e., after the solvent had become saturated with respect tothe p,p' and o,p' isomers. In each case, solvent was recycled andmake-up solvent was added as indicated:

Solvent in- M ech.

DDT Total DDT Solvent Total and Bulk in in Wet Out, Out, Evap. B. P.,ensity, Filtrate; Makeg. g. Out, g. g. Los 0. lbs./

g. Up, g. g. Cu. Ft.

moisture present in the molten DDT and remaining in 2 the product. Whenthe molten DDT is substantially anhydrous, a more or less fluffy productof low bulk density is obtained. As the amount of moisture in the moltenDDT is increased up to about 0.5 percent, a product of progressivelyhigher bulk density is obtained. It will be apparent that this practicalway of controlling bulk density is an important factor in producing DDTto meet predetermined specifications.

The following example will serve to show a typical procedure forpreparing finely divided, solid DDT in accordance with my invention, butit is to be understood that the example is given byway ofillustrationand not of limitation.

Example I A solvent mixture was employed in which methyl alcohol andacetone were present in the proportions of 100 to parts by volume. Aftereach consecutive batch the solvent was successively re-used. The moltenDDT that Was used had a setting point of 90.0 C.

Each batch was started by placing the solvent in a glass bottle providedwith a stirrer and cooled from outside by a water bath of 15 C. Toapproximately 450 grams of the solvent molten DDT was slowly poured inunder a vigorous agitation. The temperature of the mixture wasmaintained at 18 C.- C. The mixture was then poured into a centrifuge offive-inch diameter, revolving at 2700 R. P. M. After the pouring intothe centrifuge was completed, the centrifuging was continued for tenminutes. At the end of this centrifuging the product contained aboutseven to nine percent of the solvent. This residual solvent was removedby air-drying before determination of setting point and bulk density ofthe product. p

The solvent (mother-liquor) from the centrifuge was collected andre-used in the successive batch. As and when necessary, make-up solventwas added to maintain the desired volume of solvent and it should benoted that in the successive cycles no effort was made to be certainthat the entire reaction mixture was transferred in each instance to thecentrifuge. Thus, a small amount The results in the foregoing tabulationindicate a general improvement in the setting point, due in part to theremoval of traces of chlorobenzene present in the starting DDT. Thevariation bulk density indicates a variation in the moisture content ofthe successive charges of DDT. By proper control of the moisture contentof the molten DDT at a definite amount, preferably below about 0.5percent, substantially uniform bulk density can be obtained insuccessive batches.

When operating the process as a continuous process, a practical way ofcontrolling the temperature is to suitably cool the organic liquid beingrecycled so that as it is combined with the molten DDT in the desiredproportions, the resulting mixture will have the temperature desired inthe mixing chamber.

Various changes and modifications in the procedures herein disclosedwill occur to those skilled in the art and to the extent that suchchanges and modifications fall Within the purview of the appended claim,it is to be understood that they constitute part of my invention.

I claim:

The process for improving the quality of commercial DDT, which initiallycontains a quantity of isomers other than the desired p,p' isomer ofDDT, that comprises mixing together with agitation molten DDT and anorganic liquid consisting of methanol containing about 15% of its volumeof acetone, until a supersaturated solution of DDT in the liquid isobtained, and continuing to add molten DDT with agitation to therebyeffect a simultaneous dissolution of additional molten DDT in thesupersaturated mixture while maintaining a temperature of the mixturebetween 0 C. and C. and a separa tion of finely divided solid DDTtherefrom, and removing the finely divided solid DDT from thesupersaturated mixture and drying the same to thereby obtain a DDTproduct having an enhanced setting point due to the increased proportionof p,p' isomer therein.

References Cited in the file of this patent Mosher et al.: Industrialand Engineering Chemistry, vol. 38, page 922 (1946).

